Moved this to WebAdvisor.
No you don't have a virus, WebAdvisor was merely asking, as it will every time a new version installs, or if your browser undergoes a major update, if you would like to restrict your searches using Secure Search. Doing so makes Yahoo Search the default search engine in all your browsers as Yahoo is the only one to sign on to that feature, thus far.
You can always undo it in WebAdvisor's settings. Just click to the right of the cogwheel icon near the top right of any page in Internet Explorer and select WebAdvisor - settings and undo it there for all browsers. (The setting in IE controls the setting in all browsers and while there you can alter the other settings).
As Ex_Brit has said, you don't have a virus.
As you've said, WebAdvisor is already installed and enabled - this pop-up (sometimes called a "toast") is asking how you want WebAdvisor configured. Specifically, it's asking you if you want to enable SecureSearch (which it will do every few months).
A couple of minor corrections on what Peter has said;
Hope this helps